In Rust, it is common to provide different representations of a type for
different use cases. For instance, storage location and management for a
value can be specifically chosen as appropriate for a particular use via
pointer types such as Box<T> or Rc<T>. Beyond these generic
wrappers that can be used with any type, some types provide optional
facets providing potentially costly functionality. An example for such a
type is String which adds the ability to extend a string to the basic
str. This requires keeping additional information unnecessary for a
simple, immutable string.

These types provide access to the underlying data through references
to the type of that data. They are said to be ‘borrowed as’ that type.
For instance, a Box<T> can be borrowed as T while a String
can be borrowed as str.

Types express that they can be borrowed as some type T by implementing
Borrow<T>, providing a reference to a T in the trait’s
borrow method. A type is free to borrow as several different types.
If it wishes to mutably borrow as the type – allowing the underlying data
to be modified, it can additionally implement BorrowMut<T>.

Further, when providing implementations for additional traits, it needs
to be considered whether they should behave identical to those of the
underlying type as a consequence of acting as a representation of that
underlying type. Generic code typically uses Borrow<T> when it relies
on the identical behavior of these additional trait implementations.
These traits will likely appear as additional trait bounds.

If generic code merely needs to work for all types that can
provide a reference to related type T, it is often better to use
AsRef<T> as more types can safely implement it.

As a data collection, HashMap<K, V> owns both keys and values. If
the key’s actual data is wrapped in a managing type of some kind, it
should, however, still be possible to search for a value using a
reference to the key’s data. For instance, if the key is a string, then
it is likely stored with the hash map as a String, while it should
be possible to search using a &str. Thus, insert needs to
operate on a String while get needs to be able to use a &str.

The entire hash map is generic over a key type K. Because these keys
are stored with the hash map, this type has to own the key’s data.
When inserting a key-value pair, the map is given such a K and needs
to find the correct hash bucket and check if the key is already present
based on that K. It therefore requires K: Hash + Eq.

When searching for a value in the map, however, having to provide a
reference to a K as the key to search for would require to always
create such an owned value. For string keys, this would mean a String
value needs to be created just for the search for cases where only a
str is available.

Instead, the get method is generic over the type of the underlying key
data, called Q in the method signature above. It states that K
borrows as a Q by requiring that K: Borrow<Q>. By additionally
requiring Q: Hash + Eq, it signals the requirement that K and Q
have implementations of the Hash and Eq traits that produce identical
results.

The implementation of get relies in particular on identical
implementations of Hash by determining the key’s hash bucket by calling
Hash::hash on the Q value even though it inserted the key based on
the hash value calculated from the K value.

As a consequence, the hash map breaks if a K wrapping a Q value
produces a different hash than Q. For instance, imagine you have a
type that wraps a string but compares ASCII letters ignoring their case:

Can CaseInsensitiveString implement Borrow<str>? It certainly can
provide a reference to a string slice via its contained owned string.
But because its Hash implementation differs, it behaves differently
from str and therefore must not, in fact, implement Borrow<str>.
If it wants to allow others access to the underlying str, it can do
that via AsRef<str> which doesn’t carry any extra requirements.